In recent years, walk-through metal detectors have become a commonly utilized piece of security equipment. While most people are familiar with, and accustomed to the use of such detection systems in airports, the state of society today has unfortunately necessitated the use of walk-through metal detection systems in such unconventional locations as schools and courthouses. Regardless of place of use, the primary continuing function of walk-through metal detection systems is to accurately detect the presence of hidden illicit metal objects such as firearms and knives on the body of an individual.
The operation of prior art walk-through detection systems for the purpose of detecting illicit metal objects, however, has been less than satisfactory in at least four ways. First, the detection electromagnetic field generated by prior art transmit-receive coils in walk-through detection systems is plagued by the presence of weak or "dead" spots through which a person may pass an illicit metal object without detection. Alternatively, the detector may be set to respond to objects in the weak or "dead" spots, but unfortunately this causes the detector to be hyper-sensitive in the areas with normal response to trigger undesired metal alarms. Second, the prior art electronic systems provided for processing the signals output from the transmit-receive coils lack the capability of discriminating between illicit and permissible metal objects. Thus, large metal buttons, pocket change and belt buckles are often identified, quite to the annoyance of the person being scanned, as illicit metal objects. Third, the prior art electronics systems for metal detectors have no provision for discriminating against the cumulative sum total metal mass of small, permissible metal objects versus the metal mass of a single, large, illicit metal object. This drawback is the largest source of undesired metal alarms, thus necessitating additional security personnel to perform time consuming searches. Fourth, the prior art electronic systems further lack the capability of approximately identifying on the body of an individual the location of the carried metal object triggering the alarm. Thus, security agents often require a near complete disrobing of the scanned individual to locate the offending metal object and determine whether the object is illicit.
Accordingly, there is a need for an improved walk-through metal detection system that provides for a more uniform generation of the detection electromagnetic field, is capable of discriminating between illicit and permissible metal objects, is capable of discriminating between cumulative and lumped metal mass, and is capable of identifying the approximate location of the carried metal object triggering the alarm.